In various types of electronic instruments in which weak signals are highly amplified for further processing, electromagnetic interference from nearby power lines and equipment can often result in significantly contaminated signals. Such contamination frequently arises in the recording of neural signals from the surface of the human body (particularly signals from the brain and spinal cord), with the interference signal sometimes being as large or larger than the signal of interest. The interference typically contains the line fundamental frequency (60 Hz in the U.S. or 50 Hz in Europe) and the first few harmonics of the line frequency.
Many existing biomedical monitoring systems, such as electroencephalographic (EEG) monitoring systems, include optional "notch filters" which reduce this power line frequency interference by filtering out all energy at the fundamental frequency, and in some cases at the major harmonics. However, users of the instruments usually prefer to avoid using the notch filters because the filters introduce significant phase distortion near the notch frequencies and frequently cause the filtered signal to "ring" when a transient or impulse-like input is applied. A further disadvantage of the use of notch filters is that because they eliminate virtually all energy at and near the notch frequency, important signal components can be lost.